1. Field of the Invention
The present invention relates to soil excavation using supersonic nozzles, in particular to a method and apparatus for soil excavation using supersonic pneumatic nozzle with wear tip and supersonic pneumatic nozzle with wear tip for use therein.
2. Background Information
U.S. Pat. No. 5,782,414, which is incorporated herein by reference, notes that it has been well known that compressed air released in close proximity to and directed toward the ground can result in loosening of a number of types of soil. A pneumatic soil excavation tool, also called a wand, consisting of a valve, length of pipe or tubing, and ending in a reduced sized nipple or nozzle, supplied with air from a standard portable compressor, is commonly used for the purposes of dislodging soil safely from around underground utilities such as gas, water, or sewer pipes and electric, telephone, television, or other cables. The compressed air does not pose a hazard of damaging the buried utility as does a pick, digging bar, spade, bucket, or blade.
The ability to unearth safely other types of buried objects is also important. For example, in the industrial or nuclear energy sectors, such objects include glass bottles, cardboard or wood boxes, metal or fiber drums, or metal cylinders of chemical or radioactive waste. From the military sector, objects include all types of unexploded ordnance or chemical munitions.
A number of tools have been marketed produce an air stream for improved digging purposes by making the air exit the tool at a supersonic speed. For example, U.S. Pat. No. 4,813,611, which is incorporated herein by reference, discloses a compressed air nozzle for use in soil excavation to uncover buried pipes, electrical cables and the like. U.S. Pat. No. 5,170,943 discloses a similar tool with a handle, valve, electrically insulating barrel, and a nozzle. The '943 patent includes a conical shield to protect the operator, but nothing to protect the nozzle. U.S. Pat. No. 5,212,891 discloses a further excavating pneumatic nozzle design.
Air excavation nozzles should not be confused with the rocket nozzles. Supersonic air excavation nozzles used for excavation purposes are different than rocket nozzles in a number of important ways. Supersonic air nozzles for earth excavation operate at significantly lower pressures and temperatures than rocket nozzles. For example, a rocket's chamber pressure may reach 1,000 to 3,000 psig and the exhaust gas temperature may be 1,800° to 7,700° F., while typical gas jet excavation nozzles operate at around 100 to 200 psig and at 80° to 140° F. The velocity of the exhaust gas exiting from a chemical rocket's nozzle may be from 6,000 to 14,000 ft/sec; while for an excavation nozzle typical values are from 1,700 to 2,000 ft/sec. The specific nozzle profile for a typical rocket nozzle is, thus, significantly different in shape than for an air excavation nozzle.
U.S. Pat. No. 6,845,587 describes the practices of revival woody plants that are in decline, which is usually preferred to replanting. Revival avoids costs for removal and additional costs for replacement. Typically, revival has meant either aggressively fertilizing the subject plant and/or loosening the soil. Revival success is dependent on the degree of soil compaction and existing moisture content. Earlier methods include laboriously exposing roots using trowels and small digging implements. Once exposed, the roots were reburied with new loose soil or covered with the existing soil now more loosened. This early, labor intensive method is similar to the way archeologists dig for shards of pottery—slow and tedious. An improvement over manual excavation is a vertical mulching technique where a grid of 1 to 2 inch holes is drilled in the rooting soil. The holes are then backfilled with porous material and/or fertilizer.
One technique of soil loosening uses compressed air. Compressed air released at supersonic speed fractures the soil, with minimal damage to roots. Unlike porous soil, non-porous matter, such as roots, remain minimally damaged by the compressed air. Soil fracturing avoids the problems of mechanical excavation.
Fracturing soil by using compressed air is popularly used on lawns and turfs, such as golf courses. To maximize efficiency compressed air is injected in a grid. The grid is spaced so to aerate the soil evenly throughout a specified area by fracturing the soil.
Specifically U.S. Pat. No. 6,845,587 provides for the provision of a method of improving the rooting soil of a woody landscape plant comprising the steps of exposing a root collar of a plant; defining a first improvement zone encompassing the root plate area; excavating the first improvement zone with an air excavator; and adding a beneficial treatment to the first improvement zone.
The above description illustrates the growing applications for pneumatic supersonic soil excavation tools. However, the observation and analysis of damage to the exterior of various supersonic nozzles, particularly the relatively rapid failure of nozzles used during excavation of the ground, has demonstrated a need for improvement. The damage to the nozzle exterior is best described as erosion, presumably as the result of back flow of hard particles in the soil that impact the nozzle exterior with sufficient velocity and hardness to wear away (erode) the nozzle exterior. This blow back does not erode the nozzle expansion exit because the air jet coming from the nozzle expansion exit is the highest velocity in the nozzle region, and any nearby rebounding air/particles are simply drawn into the exiting air stream before it/they can reach the nozzle expansion exit. But the backflow air, when it contains sufficiently hard particles, and sufficient velocity, can and will erode the nozzle exterior.
The supersonic exit stream from the nozzle begins losing velocity, and thus digging effectiveness, as soon as the stream leaves the nozzle exit. Thus the typical digging function is performed by placing and keeping the nozzle exit, as close as possible, to the ground being excavated. This, of course, also keeps the nozzle exterior as close as possible to any high velocity back flow or blow back. When this back flow contains particles of sufficient hardness to erode any typical metal, such as stainless steel, anodized aluminum, brass etc., it is a matter of relatively brief time (e.g., days or weeks) to nozzle failure.
Experience shows that materials as hard as ordinary sand are very effective in eroding metals. Consequently this effect may also be termed as “reverse sand blasting”. The inventors of this application have experienced that this effect is seen at its worst when working in sand, in places such as middle eastern desserts. However the effect is perceptible in any soil that has sufficient content of such hard particles. Thus the occurrence and extent of the problem is difficult to predict. For, example the inventors of this application have also experienced this reverse sandblasting nozzle failure effect when working with air excavation tools in areas such as Ohio, many miles from the nearest large body of water, where one might ordinarily expect sandy beaches. Many geologic conditions can lead to soils containing small hard particles, similar to sand. An example is long term wind or water erosion of rock. It is believed that any hard particles in the soil will increase the reverse sand blasting effect on the nozzle.
Typical supersonic nozzle designs, as evidenced in the above cited patents usually focus on the interior of the nozzle design, in part because of the difficulty of these designs, and their tendency to be sophisticated, and the exterior has been left to the casual discretion of the designer. In some cases, the exterior design has been the subject of design patent protection, see for example U.S. Design Pat. No. D408,830, while there has been a functional need for a more utilitarian approach to nozzle exterior construction lurking in the soil.
FIG. 1 is a reproduction of an isometric figure from issued U.S. Design Pat. No. D408,830 and is an accurate representation of a commercially available air excavation nozzle that has been used for many years as the exterior of a supersonic nozzle used in excavation. This nozzle design is emblematic of the undesirable characteristics that the present invention solves. The integral nozzle tip outside diameter is smaller then the body of the nozzle, which exposes that nozzle body to reverse sand blasting erosion. This resulting nozzle body flat presents a perfect reverse sand blasting target. A similar perfect target is presented at the exterior end of the wrench flats that precede the trailing balance of the nozzle body. Further, the rear end of this nozzle is blunt, thereby presenting a likely snagging surface as the nozzle is withdrawn from soil.
FIGS. 2A and 2B illustrate a commercially available prior art supersonic pneumatic nozzle that was put in service with out any of the protective features of the present invention. The nozzle was used in shallow trenching in sandy soil. The nozzle tip was integral with the nozzle body. The extent of the actual reverse sand blasting erosion to the wear tip and the nozzle is illustrated by tightly spaced shading. This erosion was sufficiently severe within a month to carry the erosion through the nozzle exterior into the nozzle interior, near the nozzle entrance, as shown. In other words, nozzle failure occurred within a month of active service in a sandy environment.
It is an object of the present invention to provide a supersonic air excavation nozzle that alleviates at least some of the above stated problems associated with reverse sand blasting.